The present invention relates to a video special effect signal generating apparatus used in a television studio and, more particularly, to a control system for controlling the phase or peak position of a parabolic wave used for producing a circular or elliptic montage pattern on the screen of a television cathode ray tube.
As one of the known video special effects is known the wipe or montage effect in which into part of the picture screen a picture is inserted which is different from a picture on the remaining part of the screen. In order to obtain such video special effect, it is necessary to form a switching signal or a keying signal to combine two video signals into a single composite video signal. In a conventional video special effect producing apparatus for providing a circular montage pattern of a typical montage pattern, horizontal and vertical parabolic waves with the horizontal and vertical scanning frequencies are formed by using horizontal and vertical drive pulses, respectively and then these parabolic waves are mixed. The mixed signal is sliced by a slicer circuit to produce the keying signal. In case where the vertical and horizontal parabolic waves have each a symmetrical waveform a keying signal is produced for providing a circular wipe pattern centered at the center of a screen. The size of the circular wipe pattern is adjusted by changing the slice level in the slicer circuit through an operation of a fader control device. The vertical and horizontal shift or movement of the wipe pattern is carried out by adjusting the relative phase or time between the horizontal and vertical drive pulses through position control devices connected to phase adjusting circuits for the horizontal and vertical drive pulses. The phase or the peak position resulting from this operation shifts with respect to that of the parabolic wave for forming the pattern centered at the center of the screen.
As just mentioned, the wipe pattern is moved by adjusting the relative phase between the horizontal and vertical drive pulses. For this reason, when the center of the wipe pattern is far away from the screen center, it will be observed that an undesired twin pattern appears at the location of the screen opposite to that of a desired pattern, if the pattern is expanded by means of the fader control device. For prohibiting such an undesired twin pattern appearance, it is common practice to remove part of a parabolic waveform within one cycle period of the horizontal and/or vertical parabolic waves. The parabolic wave discontinuous during one cycle period thus obtained, however, forms a distorted pattern. In the case of the circular pattern, a part of the circle becomes linear depending on the position and size of the circular pattern. Further, as the pattern center is distanced from the screen center, it is more difficult to change over a picture from one to another.
These problems are similarly found in the case of an elliptic wipe pattern which is formed by changing the relative amplitude between the horizontal and vertical parabolic waves.
This necessitates horizontal and vertical parabolic waves of which instantaneous amplitudes continuously change even if the peak is positioned at any point of one cycle period.